Growth characteristics and whole body carbon (C), nitrogen (N), and phosphorus (P) concentrations were examined for the southern house mosquito, Culex quinquefasciatus Say, and Culex tarsalis Coquillett, reared on chemostat-grown bacteria, Pseudomonas aeruginosa. Whole body percentage of C, N, and P of Cx. quinquefasciatus larvae did not differ significantly across three bacterial concentrations (1, 5, and 10 mg of dry mass/liter) and two bacterial quality treatments (culture medium containing 5 μM P versus 50 μM P); whereas the P content of Cx. tarsalis larvae differed between the bacterial quality treatments. Low concentrations of high or low P bacteria decreased mass-specific growth rate (MGR), whereas intermediate and high bacterial concentrations affected MGR asymmetrically, depending on species. High concentrations of P-rich bacteria enhanced the growth rates of Cx. quinquefasciatus larvae relative to growth on the low P diets. Cx. tarsalis larvae reared on low P bacteria grew ≈3- to 4 times faster than larvae reared on high P bacteria. The observed asymmetric response in MGR may have been because of differential tolerance in larvae to putative toxins present in P. aeruginosa and may provide one reason why Cx. tarsalis larvae are not found in hypereutrophic aquatic habitats.